Recently, there has been an increasing interest in energy storage technology. Batteries have been widely used as energy sources in the fields of cellular phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them. In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Recently, in the development of such batteries, designs of new electrodes and batteries to improve capacity density and specific energy are mainly studied.
Among currently used secondary batteries, lithium secondary batteries developed in early 1990's have a higher drive voltage and a much higher energy density than those of conventional batteries using a liquid electrolyte solution such as Ni-MH batteries, Ni—Cd batteries, and H2SO4—Pb batteries. For these reasons, the lithium secondary batteries have been advantageously used. However, such a lithium secondary battery has disadvantages in that organic electrolytes used therein may cause safety-related problems such as ignition and explosion of the batteries and that processes for manufacturing such a battery are complicated. Recently, lithium-ion polymer batteries have been considered as one of the next-generation batteries since the above disadvantages of the lithium ion batteries are solved. However, the lithium-ion polymer batteries have a relatively lower battery capacity in comparison to the lithium ion batteries, and its discharging capacity is insufficient at low temperature. Thus, it is urgent to solve these disadvantages of the lithium-ion polymer batteries.
Such electrochemical devices have been produced from many companies, and battery safety characteristics are different in the electrochemical devices. Accordingly, it is important to evaluate and ensure the safety of the electrochemical batteries. First of all, malfunction of the electrochemical device should not cause any damage to users. For this purpose, the Safety Regulation strictly regulates ignition and explosion in the electrochemical devices. In the safety characteristics of the electrochemical device, overheating of the electrochemical device may cause thermal runaway, and explosion may occur when a separator is pierced. In particular, a polyolefin-based porous substrate commonly used as a separator of an electrochemical device shows extreme thermal shrinking behavior at a temperature of 100° C. or above due to its inherent characteristics and its manufacturing processes such as elongation, which may cause an electric short circuit between positive and negative electrodes.
In order to solve the above safety-related problems of the electrochemical device, Korean Patent Registration No. 10-0727248 and No. 10-0727247 disclose a separator 10 having a porous coating layer formed by coating at least one surface of a porous substrate 1 having many pores with a mixture of inorganic particles 3 and a binder polymer 5 (see FIG. 1). In the separator, the inorganic particles 3 in the porous coating layer formed on the porous substrate 1 serve as a kind of spacer that keeps a physical shape of the porous coating layer, so the inorganic particles 3 restrain thermal shrinkage of the porous substrate when the electrochemical device is overheated. In addition, interstitial volumes exist among the inorganic particles, thereby forming micro pores.
As mentioned above, the porous coating layer formed on the porous substrate contributes to the improvement of safety. However, due to the introduction of inorganic particles, the life span of an electrochemical device, particularly a high temperature cycle or storage life, may be deteriorated. The above documents disclose various kinds of binder polymers and their combinations, but they do not specifically disclose any binder polymer combination solving the above problem.
Meanwhile, the inorganic particles of the porous coating layer may be disintercalated due to the stress occurring during the assembling process of an electrochemical device such as a winding process. The disintercalated inorganic particles act as a local defect of the electrochemical device, thereby giving a bad influence on the safety of the electrochemical device. Thus, more endeavors for solving this problem are demanded.